The invention relates to active circulation control of aerodynamic structures. More specifically, the invention relates to active circulation control for a aerodynamic structures, such as a wind turbine blade or a gas turbine blade, by using unsteady or oscillatory flow either via synthetic jets or pulsed jets to improve lift, flow-turning characteristics, or the handling of off-design conditions, as compared to the same blade without active circulation control.
Referring now to FIG. 13, an airfoil or aerofoil 200 is the shape of a wing or blade (of a propeller, rotor or turbine) or sail as seen in cross-section. Some terminology associate with the airfoil 200 will be described. The mean camber line 206 of an airfoil 200 is a line drawn midway between the upper and lower surfaces 202, 204, respectively. The chord line 208 is a straight line connecting the leading and trailing edges 210, 212 of the airfoil 200, at the ends of the mean camber line 206. The chord length 214 is the length of the chord line 208 and is the characteristic dimension of the airfoil section. The maximum thickness 216 and the location of maximum thickness are usually expressed as a percentage of the chord length 214. Similarly, the maximum camber 218 and the location of maximum camber are usually expressed as a percentage of the chord length 214. For symmetrical airfoils, both mean camber line 206 and chord line 208 pass from center of gravity of the airfoil 200 and touch at the leading and trailing edges 210, 212 of the airfoil 200. The aerodynamic center is the chord wise length about which the pitching moment is independent of the lift coefficient and the angle of attack 220, which is the angle between the chord line 208 and the vector representing the relative motion between the airfoil 200 and the air (indicated by the arrows). The center of pressure is the chord wise location about which the pitching moment is zero.
An airfoil-shaped body moved through a fluid produces a force perpendicular to the motion called lift. Subsonic flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, often with asymmetric camber. Airfoils designed with water as the working fluid are also called hydrofoils.
A fixed-wing aircraft's wings, horizontal, and vertical stabilizers are built with airfoil-shaped cross sections, as are helicopter rotor blades. Airfoils are also found in propellers, fans, compressors and turbines. Sails are also airfoils, and the underwater surfaces of sailboats, such as the centerboard and keel, are similar in cross-section and operate on the same principles as airfoils. Swimming and flying creatures and even many plants and sessile organisms employ airfoils; common examples being bird wings, the bodies of fishes, and the shape of sand dollars. An airfoil-shaped wing can create downforce on an automobile or other motor vehicle, improving traction.
The effect by which a fluid jet attaches itself to an adjacent surface and remains attached was initially observed by Henri Marie Coand{hacek over (a)}, after whom the effect was named. “Coand{hacek over (a)} effect” is capable not only of attaching a free jet to a surface, but can also enable a tangential jet to negotiate and remain attached to a highly curved wall. The effect produces very strong entrainment of the surrounding fluid, independently of whether the external fluid is moving or stationary, and significantly reduces the surface static pressure under the jet. The point at which the flow separates from a curved surface in a two dimensional case can be controlled by the jet blowing momentum. The detailed physics of the effect are still not wholly understood.
The Coand{hacek over (a)} effect has important applications in various high-lift devices on aircraft, where air moving over the wing can be “bent down” towards the ground using flaps and a jet sheet blowing over the curved surface of the top of the wing. The bending of the flow results in its acceleration and as a result of Newton's Third Law pressure is decreased; aerodynamic lift is increased. The flow from a high speed jet engine mounted in a pod over the wing produces enhanced lift by dramatically increasing the velocity gradient in the shear flow in the boundary layer. In this velocity gradient, particles are blown away from the surface, thus lowering the pressure there.
The effect was first implemented in a practical sense during the U.S. Air Force's AMST project. Several aircraft, notably the Boeing YC-14 (the first modern type to exploit the effect), have been built to take advantage of this effect, by mounting turbofans on the top of wing to provide high-speed air even at low flying speeds, but to date only aircraft has gone into production using this system to a major degree, the Antonov An-72 “Coaler.” The McDonnel Douglas YC-15 and its successor, the Boeing C-17 Globemaster III, also employ the effect, though to a less substantial degree. The NOTAR helicopter replaces the conventional propeller tail rotor with a Coand{hacek over (a)} effect tail.
An important practical use of the Coand{hacek over (a)} effect is for inclined hydropower screens, which separate debris, fish, etc., otherwise in the input flow to the turbines. Due to the slope, the debris falls from the screens without mechanical clearing, and due to the wires of the screen optimizing the Coand{hacek over (a)} effect, the water flows though the screen to the penstocks leading the water to the turbines. The Coand{hacek over (a)} effect is also used to make automotive windshield washers that function without moving parts and to create pneumatic logic circuits.
If airfoils are considered, then a conventional sharp trailing edges airfoil exhibits the well known “Kutta” condition, named for German mathematician and aerodynamicist Martin Wilhelm Kutta. The Kutta condition is a principle in steady flow fluid dynamics, especially aerodynamics, which is applicable to solid bodies which have sharp corners, such as the trailing edges of airfoils. The Kutta condition states that the rear stagnation streamline must emerge from the trialing edge in order to avoid discontinuities in velocity and pressure. Hence, the circulation around the airfoil is uniquely defined by the airfoil geometry, incidence and free stream velocity. If however, the trailing edge of the airfoil is rounded, then the rear stagnation point is free to move, depending upon the other parameters. If the airfoil is an ellipse aligned at zero incidence to the free stream direction, then the upper and lower surface separation points should be located at the same chordwise station and the net circulation will be zero. If now a jet of fluid is injected tangentially into the upper surface boundary layer, near the trailing edge, the Coand{hacek over (a)} effect will entrain the boundary layer and delay the separation of the upper surface flow. This causes a net increase in the circulation around the airfoil. The momentum of the blowing jet now controls the position of the rear stagnation point; the airfoil is subject to “Circulation Control” by blowing. If the blowing jet is strong enough to discharge excess momentum into the wake, then the airfoil performs in a similar manner to one fitted with a jet flap. The lift is no longer produced solely by delaying the upper surface separation, but has a jet reaction thrust component that reduces the net lift augmentations δCL/δCμ, as shown in FIG. 14.
Circulation Control uses fluid injection to create a steady wall-jet at the proximity of a rounded surface in a blade to leverage the Coand{hacek over (a)} effect, as shown in FIGS. 15A and 15B. Circulation Control results in increased lift and systems using this principle have been conceptualized for a wide variety of applications—from aircraft wings to wind turbines. In aircraft wings applications, the Circulation Control works by increasing the velocity of the airflow over the leading edge and trailing edge of a specially designed aircraft wing using a series of blowing slots that eject high pressure jet air. The wing has a rounded trailing edge to tangentially eject the air through the Coand{hacek over (a)} effect, thus causing lift. The increase in velocity of the airflow over the wing also adds to the lift force through conventional airfoil lift production. In wind turbine applications, the Circulation Control works by increasing the flow rate by urging pressurized air into a duct and out a slot in the blade, thereby capturing the power from the wind flowing through a swept area of the wind turbine. Because conventional Circulation Control is generally accomplished by the steady injection of pressurized air, the need to provide large mass flow rates of such air has typically resulted in prohibitively large system costs.
It is therefore desirable to achieve Circulation Control around an aerodynamic structure, such as an airfoil, to minimize the loss of, or, alternatively increase lift producing capability, but to do so at a reduced system cost. This can be accomplished, for example, by reducing power requirements of a pressurized air source or removing the need for such a source altogether. More specifically, it is desirable to provide a blade, such as for using in wind turbines, turbomachinery, aerospace vehicles, and the like, that is optimized or designed to provide better load-bearing performance than other currently commercially available streamlined aerodynamic profiles.